Disorders of myelin include the hereditary leukodystrophies and cerebral palsies, as well as adult vascular, traumatic and inflammatory demyelination syndromes. To address this large and diverse group of disease, we established a cell-therapeutic approach to central remyelination, by which transplants of isolated human glial progenitor cells (GPCs) are delivered intracerebrally to neonatal recipients, which are then allowed to mature to adulthood. When the recipients are hypomyelinated mutants, such as the shiverer mouse, the transplanted cells mature largely as myelinating oligodendrocytes, and can rescue both the neurological phenotype and lifespan of the treated animals. Remarkably though, large numbers of human progenitors integrate into the recipient brains, wherein they effectively out-compete mouse progenitors, yielding mice with a substantially humanized white matter, and a major contingent of human glial progenitors - and ultimately astrocytes - in the gray matter as well. The resultant human glial-chimeric mouse brains provide us a variety of hitherto unavailable opportunities for studying human glial cells and their progenitors in vivo, including their responses to injury and disease processes that cannot be adequately modeled in vitro. In the proposed experiments, we will use these mice to assess the effects of toxic demyelination on human GPCs in vivo, so as to identify their molecular responses to injury-induced mobilization and oligodendrocytic differentiation during compensatory remyelination. In particular, we will use phenotype-specific cell sorting and gene expression analysis, to define the responses of these xenografted human GPCs to demyelination in vivo. These data, the first ever obtained specifically from human GPCs during demyelination and remyelination in vivo, should afford us fundamental new insights into the signaling events associated with remyelination, and their potentially targetable points of regulatory control. To achieve that end, we propose the following Aims: In Aim 1, we will treat glial-chimeric mice with cuprizone so as to better understand the responses of human GPCs to demyelination, assessing their mobilization, differentiation, responses to repetitive induction, as well as their thresholds for mitotic senescence and the regulatory control thereof. In Aim 2, we will examine the expression patterns of human GPCs in vivo, sorting them from chimeric shiverer mice both at baseline and in response to demyelination, so as to define those genes and pathways differentially regulated during GPC mobilization and remyelination. In Aim 3 we will compare the responses of co-resident human and mouse GPCs to cuprizone demyelination, so as to identify those shared pathways likely to be high-value targets in drug development, as well as those species-specific pathways whose investigation in mice might not predict human therapeutic outcome. Together, these experiments promise to inform our efforts to define new strategies for treating demyelinating brain or spinal cord injury. In addition, the databases to be generated in the course of this work, as freely available resources to the field should prove catalytic in advancing our understanding of remyelination in vivo.